Semiconductor workpieces are often implanted with dopant species to create a desired conductivity. For example, solar cells may be implanted with a dopant species to create an emitter region. This implant may be done using a variety of different mechanisms. In one embodiment, shown in FIG. 1, an ion source 100 is used. This ion source 100 includes a chamber 105 defined by several walls 107, which may be constructed from graphite or another suitable material. This chamber 105 may be supplied with a source gas via a gas inlet 110. This source gas may be energized by an RF antenna 120 or another mechanism. The RF antenna 120 is in electrical communication with a RF power supply (not shown) which supplies power to the RF antenna 120. A dielectric window 125, such as a quartz or alumina window, may be disposed between the RF antenna 120 and the interior of the ion source 100. The ion source 100 also includes an aperture 140 through which ions may pass. A negative voltage is applied to extraction suppression electrode 130 disposed outside the aperture 140 to extract the positively charged ions from within the chamber 105 through the aperture 140 and toward the workpiece 160. A ground electrode 150 may also be employed. In some embodiments, as shown in FIG. 1, the aperture 140 is located on the side of the ion source 100 opposite the side containing the dielectric window 125.
In an effort to improve process efficiency and lower cost, in some embodiments, the ions extracted from the ion source 100 are accelerated directly toward the workpiece 160, without any mass analysis. In other words, the ions that are generated in the ion source 100 are accelerated and implanted directly into the workpiece 160. The mass analyzer is used to remove undesired species from the ion beam 180. Removal of the mass analyzer implies that all ions extracted from the ion source 100 will be implanted in the workpiece 160. Consequently, undesired ions, which may also be generated within the ion source 100, are then implanted in the workpiece 160.
This phenomenon may be most pronounced when the source gas 170 is a halogen-based compound, such as a fluoride. Fluorine ions may react with the inner surfaces of the ion source 100, thereby releasing unwanted ions, such as silicon, carbon, and aluminum. A test was performed using BF3 as the source gas, supplied at a rate of 5.5 sccm. A RF power of 2.5 kW was applied to the RF antenna 120 to energize the source gas. The chamber walls 107 of the plasma chamber 105 were biased at 10 kV, while the suppression electrode 150 was biased at −1.5 kV. In this test, it was observed that less than half of the ion beam current was comprised of the desired boron ions. Additionally, nearly one third of the total ion beam current comprises ions created due to etching of the dielectric window 125 and the chamber walls 107 within the ion source 100 by the fluoride ions.
Therefore, a system and method which improves beam quality, particular for embodiments in which halogen based source gasses are employed, would be beneficial.